The present invention is related to antennas in general and in particular to a broad-band, electrically small radiator of the end-loaded filament antenna type that exhibits negligible backlobe, a useful gain and an improved directivity. In addition, the antenna (less its feed network) will occupy approximately the same volume as the present configurations.
Both of the above properties (low backlobe and the improved directivity), by themselves, constitute an improvement in the state of the art. More specifically, they result in higher accuracies when used in certain types of modern direction-finding (DF) systems at VHF and UHF such as, for example, in sectorless DF.
It is known, that a moderately directive, frequency-independent radiation pattern can be achieved with an electrically small L-shaped thin filament radiator, terminated in an end-loading resistor as shown and described in U.S. Pat. No. 3,605,097 assigned to the assignee of the present application. Such an end-loaded filament antenna is commonly referred to as ELFA.
As seen in U.S. Pat. No. 3,605,097, an ELFA embodies an electrically small filament. The filament may be comprised as two elongated wire sections with one section extending at an angle from a ground plane and the second section extending from the end of the first section parallel to the ground plane and terminated by a resistor coupled to the ground plane. Both sections lie generally in a plane normal to the ground plane and produce linearly polarized radiation in such plane with the radiation so produced predominating in a predetermined direction called the forward direction.
A weak characteristic of all ELFA's, however, is that good forward gain (i.e., the gain at the ground plane in the forward direction) and low back radiation are incompatible for these antennas.
While still used in a number of applications at VHF and UHF, because better antennas are simply not available, an ELFA with a reasonably good forward gain (i.e., a gain variation from 0 to -20 dB over two octaves) has approximately 8 to 12 dB back lobe, which can be detrimental to performance in certain applications.